Mutualism
by anonymously.peeved
Summary: AU. Secret technology whiz meets humanity's most powerful soldier. Both need each other's help with something. Of course, sparks fly.
1. Chapter 1

**Author's Notes: Wow. I can't believe that I'm done with the first chapter. I didn't even read this manga... ._. But I just ship these two people for some reason. Well, pardon me for any canon mistakes. I'm relying on my scarce knowledge of SnK and imagination to write this, so sorry in advance! Hope you'll like this. Here we go!**

**Disclaimer: I do not own Shingeki no Kyojin. I only own the plot of this fanfiction.**

* * *

A lone figure could be seen leaning back contemplatively, as he drummed his fingers restlessly against the arms of an expense leather sofa. There was a glass tea table before him, with a pile of papers uncharacteristically strewn over the surface.

For the first time, Levi was worried. With the battle of the Titans and humans extending into the twenty-first century, both sides had seen changes and developments take place over the last few decades – in themselves, and their enemies.

The Titans had adapted to humans' cunningness and intelligence, becoming cleverer and more cautious themselves; at the same time, the human race had been experimenting with new ways to upgrade their defense mechanisms, some notable new products being artificial intelligence, smartphones, and hydrogen bombs.

Despite all these inventions, the odds were still in the Titans' favor – they remained bigger, and more dangerous.

Levi massaged his throbbing temples. It was no good – one of his men had found faults in the remote control that controlled the humanoid robots, designed to help them with attacking Titans, in a bid to reduce the number of casualties on their side. Sure, his men would definitely have no problems slaying the disfigured giants – he had made sure that their skills were top-notch – but this attack was based on a large-scaled plan. They would be ridiculously outnumbered.

Levi shook his head. No – too many lives and risks were involved. However, if they did not make this necessary attack on the Titans, they would be at a disadvantage in the long run. The Titans were planning to migrate to other countries, where more humans would be ingested, causing even more unnecessary chaos in this already chaotic world.

Someone knocked on the door. "Corporal?"

"Enter," he said, sweeping the documents on the desk in one fluid motion.

It was Eren.

"Sir – our best mathematicians estimate that the Titans would be reaching the U.S. in a few hours or less. According to sources, there are at least a hundred," he said hurriedly.

Levi looked at him expectantly. After a few seconds of silence, Eren clenched his fists passionately. "So what do we do now? We have to solve this soon – "

"I'm expecting you to tell me that, you nitwit."

"Now's not the time to be kidding around, Corporal! People are getting eaten as we speak – "

_Knock, knock._

"Enter," Levi said monotonously, ignoring Eren's tirade about his apparent lack of concern for this matter, as he looked up for the briefest of seconds, before looking down again.

"What is it, Toore?" Levi did not even bother to hide his disdain for the security guard, earning a snort from the hot-tempered teenager standing in front of him, and an intimidated look from the frail middle-aged man.

"Sir," Toore bowed clumsily. "There's a girl outside. She's requesting for help."

Levi's eyebrow shot up. "A girl? Did she reveal her name?"

"No…she seemed rather poorly dressed, though."

"How old is she?"

"Not too young, but not too old…" mumbled the security guard, still bowing.

Levi furrowed his brows momentarily, before standing up. "Have you done the necessary checks?"

"Yes, I have."

Levi headed towards the door. "Very well. Show her in."

* * *

The frigid winter hair slapped against her cold, shivering form. Mikasa Ackerman squatted on the snowy ground, her bare feet turning numb from the contact. She was dirty from head to toes, dressed in rags that barely covered her chest and torso.

She guessed she should be lucky that she was even alive. It was a miracle that she had managed to defeat a Titan on her own, with just a rusty axe which she had swung mercilessly at the Titan. It remained unharmed at first, threatening to eat her alive as its fingers lazily encircled her skinny body and brought her closer to its mouth. Out of desperation, she found herself using an unusual amount of strength, which she did not even know that she possessed, to hurl the axe at its neck.

It snapped.

Its purplish blood splattered onto her body, her thin shirt torn into pieces. Remembering her dead parents' words about a 'safe haven' somewhere near that desolate place, she spent days crawling towards the military camp.

Mikasa looked up at the flag with the symbol of the Scouting Legion with dampened eyes.

_I'm finally here_, she thought.

But she was famished and tired. Worse still, she was shaking like a leaf – she did not even know how she survived the cold – she had to find some clothes to wear. Mikasa would be a fool to let such a good opportunity slip through her fingers.

Weakly, she called, "Hello? Is anyone there? I need help…"

Minutes later, a guy ran out to meet her. He searched through her pockets, checked her eyes for signs of a transformed human, before re-entering the camp. Mikasa was slightly afraid that he had abandoned her, but relief filled her as he returned, though with another person.

"…this is the girl that I had been referring to, Corporal."

Mikasa's eyes regarded the newcomer with distrust. He radiated chilliness, and some sort of superiority which she immediately disliked. He was leering at the other man condescendingly, his lips moving, as if saying some unpleasant things to her _savior._

Well, technically.

Mikasa felt a burst of anger. She forced herself to stand up. _Who does he think he is, anyway?_

"Please treat that kind sir with more respect, Mister," she said, eyes burning with fire. "He deserves it as much as any one of us does."

The obnoxious man, whose face had been hidden from view, turned his head around impatiently to look for the source of the noise – and their eyes met.

* * *

The white snow crunched underneath Levi's feet as he strolled leisurely towards the gates, several feet behind the man who tried to walk as fast as he could. He wondered why Toore was in so much haste – it was unlike him to be efficient. Although usually, Levi would have been glad that the man was making an effort to speed up, today was not one of those days. In fact, today was one of those days when he wished desperately that no one would disturb him, because millions of lives were in his hands and it was up to him to save them. Time was running out, and the last thing he needed was a nameless girl that would just be a hindrance.

"I don't see anyone," Levi commented drily. If this was just some elaborate joke that the security guard had come up with to mess with him, he would demote him immediately. Till now, no one had proposed any satisfactory solution to that remote control problem, and Levi was on the brink of killing himself over it.

Of course, he still looked calm and composed as he strolled alongside the shorter man. He did not how much time it took before a tall wall came into view.

They were approaching the gates.

"Ah, there she is!" the security guard cried, pointing at a figure not far away in the distance. "This is the girl that I had been referring to, Corporal."

Both stopped in their steps. Levi shot her a look, before narrowing his eyes suspiciously at the other man. "How did you know that she's a girl? The preliminary checks did not require you to identify her gender. You would not have been able to see her clearly from here."

Though it was not spoken, they both knew the implied meaning of the Lance Corporal's words: Toore had gone too far.

"I…went out," he swallowed.

"What did the rules say? You are not allowed to take a step out of the safety zone. What if the system identified the girl wrongly, and she is not a human, but a Titan in disguise?" hissed Levi, glaring at the security guard accusatorily. "You would have gotten all of us _killed_."

_And you yourself, too, _thought Levi silently, but he did not say that out loud. Toore had served them for a good twenty years, and Levi had come to appreciate his contributions, and cared for him quite a lot. He did not want Toore to get hurt just because of he let down his guard and allowed his compassion to get the better of him.

He glared at him contemptuously. Just then, he heard a soft but willful voice: "Please treat that kind sir with more respect, Mister. He deserves it as much as any one of us does."

Levi's eyes widened in surprise for a second_. Who was that_? He turned his head swiftly to one side only to look into the defiant, gray depths of a girl's eyes. They were hauntingly beautiful, and contained a trace of sorrow.

Unblinkingly, he snapped his fingers, and disappeared in an instant. Mikasa closed her eyes as a strong gust of wind blast in her direction, the filthy rags barely keeping her warm. Sand and snowflakes were harshly blown onto her face, and all she could see was their blurred outlines.

Then she felt the presence of a knife placed against her neck. Sharp and smooth, ready to pierce her skin.

"Who are you?" a deep, masculine voice rumbled. Mikasa shuddered, staring at his stoic face rebelliously.

"Not telling you…" she said, finding it hard to focus on his features any longer. It was a lot colder than usual, and her teeth chattered involuntarily.

"Speak up," Levi said harshly, pressing the knife even harder against her smooth neck, drawing blood. Mikasa looked at him thoughtfully, and slurred, "No…"

The last rush of the November wind knocked her out.

* * *

When Mikasa came round, the first thing she noticed was that she was wearing a clean set of clothes, with pretty embroideries and a sweet, flowery scent. The next thing she noticed was that there was a soft, warm blanket wrapped around her body. It took her a little too long to realize that she was not alone.

"You are awake."

A composed young man lounged on a sofa, twirling a pen absently-mindedly as he spoke to her. However, Mikasa thought that that sentence sounded more like an order rather than a statement, which made her slightly wary.

"Yes," she replied, waiting for him to introduce himself, or at least explain her situation. Mikasa could remember nothing of what had transpired before she became unconscious, but her intuition told her that it was nothing good. Still, she had to know.

The man made no move to do anything of that sort, though. After running his eyes over her body to make sure that her conditions were stable, he returned to the task at hand, cracking his knuckles as he stared at something in utmost concentration.

It seemed like he was staring at a remote control. Interesting.

Mikasa tried a different approach. "I'm sorry if I had offended you earlier…but may I know your name?" Those words sounded awkward for some reason. Mikasa decided not to dwell on it, instead focusing on the man's reactions.

His pupils became dilated. "Lance Corporal Levi."

He then returned to staring at the remote control, as if he did not hear her question at all. Mikasa sighed in frustration.

"I'm Mikasa," she said, attempting to start a conversation. That man had rescued her, given her a safe place to rest, and new clothes to wear…he could not be _too bad._

"Nice to meet you," she continued, though she was not sure if it had been appropriate. Did the raven-haired man not enjoy talking? She really did not understand what good staring at the remote control would do him. Perhaps, he was a telepath that could communicate with non-living things…

Mikasa decided to join him in this game of stare-at-the-remote-control.

"God," he finally said, slamming the remote control onto the table. "Do you hate me that much, woman? First you have to go limp in my arms, then you start pestering me like some annoying bug – please, I'd like some peace."

"What are you doing with the remote control?" asked Mikasa, her curiosity perked. She removed the blanket and staggered off the bed, plopping down next to him, as she picked up the remote control with a child-like wonder.

Before Mikasa could hold it for a long time, however, a hand roughly snatched it away from her, and squeezed her fingers until she felt like they could break.

"Do not try to cross me, woman," his voice was unusually gruff and low, scaring Mikasa a little. She whimpered, having just come round, and feeling a bit weak.

He kept his vice-like grip on her fingers, and Mikasa observed the mysterious man properly for the first time: he had short, straight jet-black hair, with a pair of sharp turquoise eyes that shone intimidatingly. He could have been seen as someone dangerous, for he had a sturdy, muscular physique that was beautiful beyond measures.

However, it all made no sense. Just why was he staring at the remote control like it was the most important thing on Earth?

"Hey…I think there's something wrong with it, you know," Mikasa said carefully. She watched his face for any flicker that might indicate change – and there it was: his eyes flashed with an unknown emotion.

"How did you know?" he asked guardedly. He did not move as she took the remote control from him. Mikasa played with it, hitting it left and right, before shrugging her shoulders.

"Dude, of course there's something wrong – you placed the batteries the wrong way!" she said lightly, as if it was the most normal thing to her. As if to demonstrate her point, she removed one of the batteries, flipped it, before inserting it into the remote control.

"There!" she said cheerfully. "It's back to normal now." She passed it back to a shocked Levi, who looked at her, confounded. He inhaled deeply, and pressed the response button – and waited.

A red light flashed. And flickered off. But the emotions on Levi's face were indiscernible. After a very long pause, he said, very softly, "Thank you. I owe you one." He tried to smile at her.

Not waiting for a reply, he left the room in a hurry. Taking a glance at his watch, he swore: he only had fifteen minutes left.

* * *

**Author's Notes: Thank you so much for reading this. Please review! I'm relying on you guys T^T More reviews = faster updates, so...**


	2. Chapter 2

**Author's Notes: Thanks for the amazing reviews! I promise I'll reply soon hehe. Here's Chapter 2. Hope you guys will like it.**

* * *

"Hey, Corporal," Mikasa shouted, watching as the door swung close. "Your remote control!"

But he was already gone. Mikasa sighed in frustration. Her eyes darted around the room, and she quickly grabbed a jacket which she guessed belonged to the Corporal.

"I'm coming with you too!"

* * *

People began muttering as the great Corporal arrived at the Headquarters with his usual air of confidence. Levi was still the old, egoistic hag – glaring at his subordinates with steely eyes.

"You fools," he jumped straight to accusations. "I can't believe that none of you solved the issue."

The attack group hung their shame shamefully, knowing what was to come: the cancellation of their initial plan.

"And guess what? A pesky girl fixed the freaking remote control," he said, rolling his eyes. "Useless brats."

Levi huffed impatiently at the audible gasps that escaped the men's mouths. When no one moved, he raised an eyebrow. "Well? What are you waiting for? Get your lazy asses up and head to your respective stations!"

Finally, his men recovered from their shock.

"That's crazy," one muttered.

"It's not…cancelled?" another asked disbelievingly.

Levi wanted to slap himself. The stupidity of his men was _astounding._

"Get it through your thick skull that a girl – a weak, pathetic girl – actually saved your asses." Levi looked at them disapprovingly. "How disappointing. I expected you to do better than that."

He turned and left, brushing off the speck of dust on his collar. Then he saw someone who was not supposed to be here – _Mikasa something_, Levi thought, furrowing his brows.

"Corporal - " she began, running towards him at an extremely fast speed. Her face was sweaty, tainting an orange leather jacket – wait, was that his? - with her dirty germs. Levi stared at her, unimpressed, as she stopped right before him with an excited smile on her face.

"Hello, Corporal. I came – "

Before she could finish her sentence, a man pointed at her. "Watch out!" he cried, aghast. "There's a strange girl!"

Without stopping to examine the situation, the armed men grabbed their firearms and aimed them at Mikasa, remembering the usual procedures drilled into their brains.

"Exterminate!"

It was like in slow motion. Levi could feel the hundreds of bullets flying in the same direction, towards the same target. He did not dare to breathe, as they moved agonizingly slow – and he blinked.

That moment was over. The darts zoomed past Levi's face and a second later, he heard a loud clatter. For some reason, his heart sank.

_What a waste_, he thought. _That Mikasa woman might actually prove to be more useful than those oafs._

"What the fuck?" he snapped at them. "Who told you to fire?"

But his men were not listening to him. Their jaws hung agape, as they stared at something, looking extremely…disconcerted.

"How is it even possible…" the voice trailed off, breaking the tense silence. Levi wondered if his harsh words discouraged them so much that they looked disturbed by the fact that they were bright enough to use firearms.

Levi twitched. "Quit staring. Nothing can be done now. You still have shit to do." When they still kept up the act of gaping stupidly, the Corporal had had enough.

"Look, I'll feed you to the Titans if you don't start doing – "

What.

_What._

Levi made the mistake of looking in the same direction.

He also found himself staring in disbelief. There the Mikasa woman stood, holding up a shield nonchalantly, as if hundreds of men did not simultaneously fire at her. What was more incredible was that all those bullets were glued to the shield, as if there was an unbreakable force holding them together.

A few moments passed, and Mikasa looked between the men and the Corporal awkwardly. She was not used to having so much attention drawn to her.

"Uh – " she said, taking small steps towards Levi. "You forgot the remote control."

"Oh. Thanks," Levi said, looking at the object in his hand, dazed. He still could not believe what had just happened. Feeling the heat of the girl's gaze on him, he schooled his features into that of composure.

"Care to explain, brat?" he drawled. "I only have five minutes left before the scheduled attack."

There was it again – the bored look on her face, as if all those things that happened actually made any sense at all.

"I have a super-magnet with me," she answered. "It is very strong. One stroke was all that it took to turn this," she gestured towards the shield, "into a magnet. I was pretty lucky, actually; if those bullets were made of bronze, I would have been dead meat."

Levi hated looking surprised, but he really could not help himself. What on Earth was this magnet thing? He had never heard of it in his life.

He looked at his watch yet again, and snapped his fingers, bringing his soldiers back to reality. They had spent way too much time gawking at her. "Listen up, idiots – you still have shitloads of Titans to face later. Get yourselves prepared in thirty seconds. We are going off one minute later."

Levi tilted his head slightly, glancing at Mikasa out of the corner of his eye.

"Welcome to the club, woman," he said. "You're in."

"Thanks?"

* * *

"Ackerman," commanded Levi impatiently. "Drop two bombs now!" The aircraft turned sharply in the air as a robust arm made a grab for it. Mikasa jammed her foot on a pedal, cursing loudly.

"I know, _Corporal_," she seethed. "I have a brain for God's sake!" Levi's head was smashed roughly against the window as the two-men battle plane dove to dangerously low heights.

"Look where you are going, woman!" he yelled, pushing her to one side as he grabbed the steering wheel and maneuvered them up into the air. His frame crushed hers, and Mikasa angrily pushed him back into his seat.

"Dude, I know what I'm doing!" she exploded, deliberately making a flip in the air just so the Corporal's head could crash into something else, even if just for a few seconds. Her eyes gleamed triumphantly, and she wondered why she enjoyed pissing him off so much.

"That was payback for just now."

Levi rubbed his sore head, and glared at Mikasa, refusing to back down. "You wanna disrespect me, huh? Come, I'll show you!" he hissed, tugging her body forcefully out of the driver's seat, and jumping into the vacant space. He turned the steering wheel to the left until it reached its extreme, and a second later, Mikasa found herself dangling in the air, before falling back to the seat with a loud 'plomp'.

"Asshole!" Mikasa cried, realizing that the Corporal had buckled up swiftly, so he was not harmed.

"Hmm?" Levi smirked, turning the steering wheel in the exact opposite direction, causing Mikasa to fall to the seat painfully once again. "How was that, Ackerman?"

Mikasa did not like that challenging glint in his eyes. Meeting his eyes with defiance, she was about to retort, when a loud voice started blaring:

"Agent Armin speaking. Can you hear me, Corporal? Over."

"Yeah," replied Mikasa before Levi could speak a word. Quickly, he added, "Over!"

"Who's that? Anyway, Team C's done with their assigned site. Awaiting your instructions, Corporal. Over."

"Noted. Thanks!" shouted Mikasa into the microphone, one hand holding back the Corporal, grinning cheekily when she saw his narrowed eyes. Slapping her hands off, Levi leaned forward, and spoke in a crude manner.

"Not over, so you had better fucking listen to me. Instruct Team C to help Team F out. If Team C suffered a lot of losses, ask them to retreat. Understood? Over."

"Understood, Sir. Over."

Levi straightened his back and shot daggers at Mikasa. "Don't try to be funny again, woman, or else I'll have you scrubbing my floors on your knees." Mikasa looked away, though Levi could tell that she was unhappy with his threat.

Whatever. Anything to make her shut up.

He finally had his desired peace, but it did not last long. He shifted in his seat to be in a more comfortable position, before he felt the plane tremble. Then, there was a thunderous explosion, its fiery clutches squeezing the air beneath the battle plane. The trembling did not stop, and Mikasa grew concerned. "Hey, Corporal – "

"Corporal, can you hear me? Team C's aircraft mysteriously exploded. Reply ASAP. Over." This was not the time for child's play, Mikasa realized, watching Levi's eyes grow dark.

"Someone's trying to fuck with us," he said softly. "Fine. We'll fuck back with them." Silence. "Ask every team to release two missiles. One of them is _bound _to kill that motherfucker. Over," his eyes darkened even more.

There was something extremely wrong, Mikasa thought, furrowing her brows as she mulled over the situation. _But why?_

"I believe in science, Corporal," she said silently. "There has to be a reason behind this explosion." Levi did not reply, though she could tell that he was thinking hard too.

"It exploded for no reason at all…" he said, holding his chin.

"Rather, it exploded for a reason, though we could not understand, nor see it," paraphrased Mikasa, trying to be helpful. She watched the blue orbs swirl, feeling herself drown in their intensity.

"That's it," Levi suddenly said, looking up. "We just can't see it! God, we are so stupid. The reason is so simple: there's an invisible Titan."

Mikasa's eyes widened at the realization. "You're right," she whispered.

"Oh shit," Levi swore, banging his head against the seat. He switched on the receiver. "Armin? Lance Corporal Levi here. There's an invisible Titan smashing our aircrafts. Tell the rest now. Over."

As soon as he's done, Levi turned to look at Mikasa expectantly. "Well, you're the technology genius here. What do we do next?" he wore a scowl, as if it hurt his ego to admit that he had to seek her help.

"I don't really know…" Mikasa said, biting her lower lip.

Levi arched an eyebrow, and Mikasa felt a pang of guilt for not being useful in critical situations. "Science does have its limitations," she finished lamely. Levi stared at her for a few seconds, before he laughed hollowly.

"So much for thinking that you are better than the rest," he said, sneering. "Looks like I've been mistaken, huh?"

"I…" Mikasa opened her mouth to defend herself, _to say something,_ but nothing came out. "Sorry."

The Corporal ignored her, stuffing his hand into the pocket to pull out the same remote control that Mikasa had fixed, and Mikasa felt out of place in that constricting space. Being so close to the man known for being the strongest in history made her feel inferior. She looked down, forcing herself to keep her apathetic façade.

"Hey," she said. "There's something we can try, though." This was her only chance to redeem herself.

The Corporal looked at her disinterestedly. "What?"

"I'll cut myself, and attract the Titan with the scent of my blood. There's an air pressure detector on this aircraft isn't there? We can use it to make a rough guess as to where the Titan is."

Levi watched the flame in Mikasa's eyes burn brightly, and slowly, a smirk formed on his face. "Why, Ackerman, not bad."

"It might not work, though," she warned, scrolling down the window, and leaning over the window frame. Mikasa knew that she had to be quick, because she could already feel the effects of the scarce amount of oxygen on her body.

"What are you doing, Ackerman? Come back!" growled the Corporal, leaning towards Mikasa and encircling her waist with his arms. "This is too dangerous!"

"Trust me!" she said, attempting to shrug him off of her. "I will be fine." She retrieved the silver dagger from her pocket, and made a large gash on her arm. Smiling, she looked at her handiwork appreciatively and flashed it to Levi.

"I won't fail you again," she said determinedly, stretching her bloodied arm out of the window. Levi looked at her, and felt his chest warming. _She's really brave_…Levi thought. _She'll make a good soldier._

Mikasa's arms remain outstretched, and her crimson blood dripped down her hand, coloring the whole arm red. Levi looked at her worriedly, despite the tough smile that she had been giving him. Her face was deathly pale, her lips slightly drained of blood, and her body felt cold.

Suddenly remembering the state Mikasa was in when he found her, Levi exclaimed, "Damn, woman, you still haven't eaten!" Mikasa pretended not to have heard him, instead focusing on the air pressure indicator near the driver's seat.

"Air pressure indicator…" she mumbled. "Look…" What was she trying to say? Levi could not really hear her over the strong wind. He turned his head to look at it, and frowned. The air pressure was slightly different from usual, and he looked to Mikasa for more details.

"Hurry…East…" she managed to choke out, barely breathing. Levi recognized the symptoms of low oxygen intake and cursed. Things were not looking good. He conveyed that message to Armin, asking him to tell everyone to get out of 5 kilometers within his radar. He stroked the red button of the remote control, knowing that the highly destructive bomb will then activate.

"…careful," said Misaka, her eyelids drooping. Levi fetched her into his arms and pulled her into the safety of the interior parts. She looked so weak, so fragile, that Levi felt his heart softening as he held her gently in his arms.

He shut his eyes and pressed the button.

Only a second too late.

A fist smashed into the engine of the aircraft, and it soon spiraled downwards towards Earth. "Shit!" Levi said, jumping out into free air while holding Mikasa. "Fucking Titan…"

He aimed multiple shots with his free hand, at the spot where he was sure the Titan was to be. The bomb would explode thirty seconds later, giving the experienced Corporal enough to land on the floor safely.

"…two…one."

The plane exploded. Levi heard a loud moan, followed by a sound of something crashing to the ground. He hugged Mikasa tightly, hoping her conditions were not too critical. He shut his eyes, and willed everything to be over soon.

Then something unexpected happened. The monsoon wind blew hard at them, causing Levi to lose his balance in the free fall. He tumbled towards the ground quickly, and Levi knew that he had no time to lose.

Mikasa tucked in his arms, Levi opened his parachute and prayed.

And everything went black.

It seemed like they were trapped on a deserted island, all alone.

* * *

**Author's Note: I don't know if this is fast or not, but it's certainly the fastest I've ever been. XD Please review! I welcome all constructive criticism. :) Thank you so much!**


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